>> Presumably CAD also allows you to do things like "find a point 3/5 
>> along the length of this edge".
> 
> AFAIK there is not usually any automatic way to do that, you'd just need 
> to add a formula to one of the point dimensions that says something like 
> "Edge1.Length * 3/5".  That way, whenever the length of that edge 
> changes, the point is always fixed 3/5 of the way along.

Right.

> You could then 
> draw some geometry referenced to that point, then of course that would 
> all move with the point too.  It gets a bit like POV, where you can just 
> define a variable for something like "screw position", and then drive 
> lots of other features like holes and cut-outs in lots of parts from 
> that one variable.  The benefit of this is when your customer says "ok 
> we need that screw moved 1.2mm" it only takes you a few seconds to 
> update all your parts rather than an hour or two.

SketchUp allows you to draw precise sizes easily, and it allows you to 
line things up easily. What it does not do is allow you to "link" things 
so that when something changes, other things change to match. You'd have 
to propogate any necessarily changes manually yourself. SketchUp makes 
it quite easy to adjust everything by exactly the right amount, but you 
still have to know what to adjust and how to adjust it.

>> the best feature of all is the way surfaces "stick" to each other as 
>> soon as they touch...
> ...
>> Also, you can't delete any item smaller than a certain size on screen. 
>> You have to zoom in first. And the minimum size is significantly 
>> larger than you'd expect.
> 
> I would expect there would be settings for these sorts of things?

The delete size, maybe. (I haven't found the setting anywhere though. 
There are surprisingly few settings you can adjust.) The sticking 
appears to be a design feature - annoying as it is. (You can prevent it 
by using layers or grouping surfaces into seperate entities.)

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